This invention relates generally to xerographic copying methods and apparatus, and more particularly, it relates to the fixing of particulate thermoplastic toner by direct contact with the surface of a fusing member having a novel fluid release surface.
In the process of xerography, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support, such as a sheet of plain paper, with subsequent affixing of the image thereto.
In order to affix or fuse electroscopic toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent into the fibers or pores of support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member. In both the xerographic as well as the electrographic recording arts, the use of thermal energy for fixing toner images onto a support member is old and well known.
One approach to thermal fusing of electroscopic toner images onto a support has been to pass the support with the toner images thereon between a pair of opposed roller members, at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the fuser roll thereby to affect heating of the toner images within the nip. By controlling the heat transferred to the toner, virtually no offset of the toner particles from the copy sheet to the fuser roll is experienced under normal conditions. This is because the heat applied to the surface of the roller is insufficient to raise the temperature of the surface of the roller above the "hot offset" temperature of the toner at which temperature the toner particles in the image areas of the toner liquify and cause a splitting action in the molten toner resulting in "hot offset". Splitting occurs when the cohesive forces holding the viscous toner mass together are less than the adhesive forces tending to offset it to a contacting surface such as a fuser roll.
Occasionally, however, toner particles will be offset to the fuser roll by an insufficient application of heat to the surface thereof (i.e. "cold" offsetting); by imperfections in the properties of the surface of the roll; or by the toner particles insufficiently adhering to the copy sheet by the electrostatic forces which normally hold them there. In such a case, toner particles may be transferred to the surface of the fuser roll with subsequent transfer to the backup roll during periods of time when no copy paper is in the nip.
Moreover, toner particles can be picked up by the fuser and/or backup roll during fusing of duplex copies or simply from the surrounding of the reproducing apparatus.
One arrangement for minimizing the foregoing problems, particularly that which is commonly referred to as "offsetting", has been to provide a fuser roll with an outer surface or covering of polytetrafluoroethylene, known by the trade name "Teflon" to which a release agent such as silicone oil is applied, the thickness of the Teflon being on the order of several mils and the thickness of the oil being less than 1 micron. Silicone oil, polydimethylsiloxane, which possesses a relatively low surface energy, has been found to be a material that is suitable for use in the heated fuser roll environment where Telfon constitutes the outer surface of the fuser roll. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to form an interface between the roll surface and the toner images carried on the support material. Thus, a low surface energy layer is presented to the toner as it passes through the fuser nip and thereby prevents toner from offsetting to the fuser roll surface.
A fuser roll construction of the type described above is fabricated by applying in any suitable manner a solid layer of adhesive material to a rigid core or substrate, such as the solid Teflon outer surface or covering of the aforementioned arrangement. The resulting roll structure is subject to wear and degradation due to continued operation at elevated temperatures and also to damage from accidental gouging by stripper fingers conventionally employed in such systems. The foregoing, in many instances, necessitates replacement of the fuser roll which is quite costly when a large number of machines is involved.
Moreover, the polytetrafluoroethylene along with the coating of silicone oil is of sufficient thickness to constitute a poor thermal conductor, and longer nip dwell and higher fuser roll temperatures are required to deliver the fusing energy required to fix toner. Also, control of the surface temperature of the roll presents a problem due to large temperature variations occurring before and after contacting of the substrate carrying the images.
Silicone elastomers have also been used on the surface of fuser members for fixing thermoplastic toners on receptor surfaces. In U.S. Pat. No. 3,669,707 issued June 13, 1972, silicone elastomers containing fluorinated organic polymer fillers of specified surface energy are used on the surface of fuser members for fixing toner materials. However, the coating is of sufficient thickness to constitute a poor thermal conductor, and longer nip dwell and higher fuser roll temperatures are required as in the case described above. Furthermore, the silicone gum filler is, of necessity, a dual component system to prevent hot offset. This in turn leads to additional preparation and handling problems.
In view of the foregoing it would appear that the high thermal conductivity and wear resistance of bare metals or similar materials would be desirable for utilization in fuser member structures and certain materials have been found which are satisfactory for such application. Commonly used release agents such as pure silicone oils and mineral oils, have been tried in combination with various metals and other high surface energy materials but with relatively little or no success. However, certain materials have been found which are satisfactory for such application. These materials, fusing methods and devices are described in Assignee's co-pending patent application Ser. No. 383,231 filed July 27, 1973, now U.S. Pat. No. 3,937,637, which includes providing a coating of a polymer release material of the type which oxidizes and thereafter is capable of reacting with the fuser surface material to form a first barrier coating portion upon the fuser member and a second replenishing release portion thereon. In Assignee's copending application Ser. No. 491,415 filed July 24, 1974, a coating of polymeric fluid containing built-in functional groups which interact with the fuser member surface to provide an interfacial barrier layer and a low surface energy film of the fluid, is provided upon a fuser member. Exemplary of the build-in functional groups in the foregoing reference are carboxy, hydroxy, epoxy, amino, isocyanate, thioether and mercapto.